1. Field of the Invention
The present invention relates to a display optical system used in an image display apparatus which is referred to as a head mounted display or a spectacle type display or the like for observing an original image formed on a reflective image forming element.
2. Description of the Related Art
An image display apparatus of a head mounted type (so-called a head mounted display) has conventionally been known in which an image forming element such as an LCD (Liquid Crystal Display) is used to enlargingly display an original image displayed on the image forming element through an optical system.
Since the head mounted display is mounted on the head of an observer, reduction in size and weight of the entire apparatus are particularly needed. Furthermore, the head mounted display is required to have small thickness in the visual axial direction of the observer in consideration of weight balance, appearance and the like. In addition, it is desirable to provide the largest possible enlarged image to produce a dynamic effect in the displayed image.
For example, in the patent documents 1 through 4 which are described below, there are proposed head mounted displays which use a transmission type image forming element and an optical element (so-called a free curve surface prism) having an incident surface, a plurality of reflective surfaces and an emergent surface including a rotationally asymmetric surface on surfaces of a transparent body to achieve a reduction in thickness of the entire apparatus.
On the other hand, advancement has been made in the refining of the image forming element such as the LCD or the like, so that an image forming element has been developed which has the same number of pixels as the conventional element while being reduced in size than the conventional element. As such image forming element which has been reduced in size and has been refined, a reflective image display element is regarded as being promising.
However, it is required to increase the magnification power of the display optical system in order to achieve the observation angle of view which is the same as the conventional one while using such size reduced image forming element. When the magnification power is increased, the display optical system can be easily increased in size.
Furthermore, when the reflective image forming element is used, it is required to provide an illumination system for allowing illumination light to be incident thereon from the side of the display surface. Consequently, the entire apparatus can be easily increased in size.
As an image display apparatus which deals with these problems, apparatuses have been proposed in the patent documents 5 and 6 which are described below.
The image display apparatus proposed in the patent document 5 has two free curve surface prisms. In the apparatus, the angle of view is widened with respect to the panel size by using one of the free curve surface prisms as a relay optical system, forming an intermediate image once and then guiding the intermediate image to the observer by using the other free curve prism as an eyepiece optical system.
Furthermore, in an the above apparatus, an optical path which is crossed in a shape of a numeric character four is formed in one of the free curve prism to reduce the size of the optical system with respect to the optical path length. Furthermore, an attempt is made to further reduce in size by using the prism as an illumination optical system in addition to using as the relay optical system as described above.
Furthermore, the image display apparatus proposed in the patent document 6 has a display optical system having three free curve prisms. In the apparatus, after an intermediate image is formed once with two free curve surface prisms which serve as a relay optical system, the intermediate image is guided to an eye of an observer by using the remaining free curve surface prism as an eyepiece optical system. As a consequence, the angle of view is widened with respect to the image forming element. Furthermore, an illumination optical system is inserted which does not contribute to the image formation between the three free curve surface prisms and the image forming element.
[Patent Document 1]
Japanese Patent Application Laid-Open No. H7(1995)-333551
[Patent Document 2]
Japanese Patent Application Laid-Open No. H8(1996)-50256
[Patent Document 3]
Japanese Patent Application Laid-Open No. H8(1996)-160340
[Patent Document 4]
Japanese Patent Application Laid-Open No. H8(1996)-179238
[Patent Document 5]
Japanese Patent Application Laid-Open No. 2000-187177 (corresponding to U.S. Pat. No. 6,222,676)
[Patent Document 6]
Japanese Patent Application Laid-Open No. 2002-90692
However, in the image display apparatus which is proposed in the aforementioned patent document 5, there is a problem in that a flexibility of the design of the illumination optical system or the display optical system is lacking and the size of the light source is easily enlarged because only one of the free curve surface prisms is used as an illumination optical system.
Furthermore, in order to reduce the size of the display optical system with respect to the optical path length, the optical path in a shape of a numeric character four is formed in one of the free curve prisms. Consequently, the structure of the prism is limited.
In addition, it has been described that after the illumination light is allowed to be incident on and transmitted through one of the optical surfaces of the prism, the display light (the light which is modulated with the image forming element) which is sent back to the surface is subjected to a total reflection, and the incident area for the illumination light and the reflection area for the display light are divided on the optical surface. However, since it is required to form the optical path formed in a shape of a numeric character four is formed in the prism including the incident surface for the illumination light, it is extremely difficult to achieve the aforementioned items by using a small size prism.
In addition, in the image display apparatus proposed in the patent document 6, the illumination optical system is inserted which does not contribute to the image formation between the three curve surface prisms and the image forming element. As a consequence, the restraint condition of the design of the display optical system is increased so that it becomes difficult to realize the larger angle of view and reduction in size.